1. Field of the Invention
This invention relates to apparatus and process for preparing calcined gypsum whose properties in general, e.g., set time, dispersed consistency, rate of strength development, and other properties are particularly adapted for use in forming a cast gypsum article, such as for use in gypsum board manufacture and in making dried bagged industrial plasters for subsequent casting usage.
In automated gypsum board manufacture using continuously calcined gypsum stucco, a large portion of the processing time and energy is devoted to removing excess water from the wet board which was required to fluidize the calcined gypsum in water and obtain proper flow of the gypsum slurry. Thus calcined gypsum made by continuous calcination may have a dispersed consistency of about 100-200 cc., for a water usage of 85-100 parts of water per 100 parts of the calcined gypsum. For purposes of the present invention, "dispersed consistency" may be defined as the water volume required to give a standard viscosity or flow when a standard amount by weight of stucco is dispersed by mechanical mixing in a laboratory mixer at high shear intensity and for a standard time to equal mixing encountered in the gypsum board forming line, e.g., 7 seconds, or an industrial plaster casting mixer, e.g., 60 seconds. While the dispersed consistency may thereby be expressed in a particular number, such number is variable from one processing line to the next depending on the particular stucco and the amount of flow for that stucco being most desirable for the particular processing line; and the value of this number resides in the relative improvement for that line and in the time of temperature rise set for the same gypsum source. "Temperature rise set" and "set suddenness" for purposes of the present invention may be defined, respectively, as the particular time in which a particular calcined gypsum completes the exothermic reaction characteristic of hydration of stucco to gypsum; and as that portion of the exothermic stucco set time curve which exhibits the maximum rate of temperature rise. While defined times and temperatures may be given, it will also be appreciated here that they are meaningless from one gypsum source to another or from one gypsum board production plant to another; and only the relative change in the time and the set time curve for a particular gypsum source are of value.
2 Description of the Prior Art
In U.S. Pat. No. 4,117,070 there is described apparatus and processes for continuously treating calcined gypsum so as to lower the water demand and provide a treated gypsum mass which may be continuously fed into the slurry mixer of an automated gypsum board line. The treatment comprises thoroughly blending small amounts of water into the dry stucco, resulting in a damp but dry appearing material and allowing it to "heal" before usage in gypsum board manufacture. That is in allowing the small amounts of free water to remain on the particle surface for 1-10 minutes while fractures on the particle fuse so as to resist subsequent disintegration into micron sized fractions on mixing with water for hydration. Material so produced is particularly suitable for immediate utilization in gypsum board manufacture; however, if such material is not used right away, the set suddenness of the material begins to vary erratically. Further, it has been found that board manufactured by such processes is considerably weaker than if untreated stucco were used in making the board. In a U.S. patent application by Eugene O'Neill entitled "Improved Process for Preparing Calcined Gypsum and Gypsum Board," Ser. No. 939,624 there is disclosed a process which is an improvement over said patent by grinding the water treated material so as to increase the surface area of the particles generally up to about 4 times in order to recapture physical and rheological properties lost by the water treatment. Such requires that the automated line from calcination of the gypsum rock source to the formation of the board be modified by the addition of at least two pieces of equipment, the water blender and the grinder; and the addition of at least two separate operational steps. That patent application teaches a "healing" time between those two operational steps. It would be desirable to accomplish the same results, but with less added equipment, less added processing time, and in a simplified procedure. For example, for a full size plant operation, the elimination of either one of the two added pieces of equipment required could save a capitalization expense of perhaps a quarter of a million dollars.